1. Field of the Invention
The present invention generally relates to ultrasonic sound systems and, more particularly, to systems for acoustically scanning optical images.
2. Description of the Prior Art
A general introduction to this field of technology can be found in an article entitled "Acoustic Surface Waves" written by Mr. G. S. Kino and Mr. H. J. Shaw and published in the "Scientific American", October 1972, Vol. 227, No. 4 at page 51.
The initial developement of acoustically scanned optical imaging systems was carried out by Mr. C. F. Quate. This early work disclosed that when a photosensitive semiconductor body was placed close to the surface of a piezoelectric acoustic surface wave delay line, a pattern of illumination incident on the semiconductor body caused a corresponding change in the interaction between an acoustic surface wave in the delay line and the electrons in the semiconductor body.
A more recent development has been the use of a convolver incorporating a lithium niobate (LiNbO.sub.3) surface wave delay line separated by an air gap from a silicon (Si) semiconductor body. The air gap is used to prevent the lithium niobate from mechanically stressing the silicon body during operation. The development and use of lithium niobate convolvers is further described in the following articles and patents:
U.s. pat. Nos. 3,826,865 and 3,826,866 entitled "Method for Acousto-Electric Scanning" issued to Mr. C. F. Quate et al. and assigned to the Board of Trustees of Leland Stanford Junior University. PA1 N. j. moll, O. W. Otto, and C. F. Quate, "Scanning Optical Patterns with Acoustic Surface Waves", J. de Physique, 33, Colloque C-6, Supplement, pp. 231-234 (November-December 1972). PA1 C. f. quate, "Optical Image Scanning with Acoustic Surface Waves", pending publication in the IEEE Transactions on Sonics and Ultrasonics. PA1 S. takada, H. Hayakawa and N. Mikoshiba, "Surface-Wave-Acoustoelectric Image Scanner", Proceedings of the 5th Converence (1973 International) on Solid State Devices, pp. 194-198, 1973.
The parametric interaction between the two counter propagating surface waves in a delay line and the theory of operation of convolvers is described in an article entitled "Signal Processing by Parametric Interactions in Delay-Line Devices" written by Mr. G. S. Kino et al. and published in IEEE Transactions on Microwave Theory and Techniques, Vol MTT-21, No. 4, April 1973, pp. 244-255. This article also refers to the development of monolithic convolvers.
Heretofore, one problem experienced with lithium niobate convolvers has been maintaining the uniformity of the spacing between the semiconductor body and the lithium niobate. This spacing across the air gap usually has a thickness of from one thousand angstroms to one micron. In most practical applications it is difficult to prevent the lithium niobate from deforming the infintesimal amount necessary to destroy the uniformity of the air gap. The problem is further compounded because the air gap itself prevents the convolver from being mechanically rigid and flexure resistant. In addition, random charges of electrons or ions tend to collect on the outside surfaces of the lithium niobate and the semiconductor body and can destroy the uniformity of the interaction.
One problem heretofore experienced with convolvers has been the large, continuous, minimum signal output occurring during operation. This large, minimum output is called the dark current because convolvers generate this large signal even when there is no incident illumination and the convolver is located in the dark. The dark current is generated because the two counterpropagating acoustic waves are mixed together within the convolver and produce a combined output signal. The illumination incident on the convolver merely varies this pre-existing output signal level.
Another difficulty with these prior convolvers has been their limited dynamic range. At the high end of the range the maximum output signal occurs when there is so much incident light that the convolver saturates and produces a continuous high level output. At the low end of the range the dark current described hereinbefore comprises the minimum, low level signal output. Any signal weaker than the dark current is buried in the low level output and is not observable. Since the dark current is indistinguishable from a signal level produced by ordinary incident illumination, there has been heretofore no desirable way to lower the minimum signal level and thereby increase the dynamic range of the system.
Lastly, there has always been a continuing need to find devices that will convert patterns of light into electrical signals and, in particular, to discover systems to perform fast Fourier and Fresnel transforms of these light patterns in real time. Further, there has always been a need to develop devices that will transform patterns of light into electrical signals with a nondestructive readout.